I would do something like this:
lib
|------ lib.py
|------ platform_1
| |------- _lib.py
|
|------ platform_2
| |------- _lib.py
and in lib.py
# this module becomes the _lib module for one platform of either 1 or 2
if platform == 1:
from .platform_1._lib import * # python 3 import
if platform == 2:
from .platform_2._lib import *